The present invention relates to a composite superconductor used for a superconducting transmission line, a variety of superconducting magnets and the like.
A composite superconductor consists of a super-conducting material and a normal conducting material, and it is finished into a round, flat or hollow type wire, for example, by burying superconducting wires of about 5 to 250 .mu.m diameter into the normal conducting material such as copper, aluminium or other metals, or alloys thereof. Having superconductivity, such a finished wire does not generate heat under a definite magnetic field and a definite electric current. However, temporal changes of an applied current bring about a loss accompanied by magnetization. If breakdown of the superconductivity should happen for some cause, a large quantity of Joule heat would be evolved.
For these reasons, a portion of the composite superconductor is utilized as a cooling face or surface which is directly in contact with a refrigerant, for example liquid helium, to cool the superconductor.
Upon a breakdown of the superconductivity, the conditions to prevent the spread of a normal conducting range evolving Joule heat can be represented by the following equation: EQU .rho..multidot.I.sup.2 /A.multidot.P.multidot.q&lt;1 (1)
where .rho. is the electrical resistivity of a normal conducting material, I is an applied current, P is the cooling perimeter, A is the cross-sectional area, and q is heat flux.
As is understandable from the above equation (1), in order to raise the electric current, it is necessary (a) to provide the normal conducting material having a low electrical resistivity .rho. and a large cross-sectional area A, or (b) to raise the cooling capability P.times.q. The former solution (a) is limited from the viewpoint of the quality of the normal conductor or of the design for the composite super-conductor, and hence, one cannot help directing the subject of his endeavors to the latter solution (b).
As one example of this countermeasure, a method is well known which comprises forming a number of rectangular sectional grooves of about 1 mm depth and about 1 mm width in the surface of a conductor coming into contact with liquid helium to increase the perimeter P. It has also been tried to provide cellulose coatings on the surface of a conductor. However, the improvement of the cooling capability is not yet sufficiently carried out.